ABSTRACT Blood cancer constitutes nearly 10% of all newly diagnosed cancer cases in the US and is the leading cause of cancer mortality for children and young adults. Leukemia is a cancer of developing progenitor blood cells, and the most common subtype of leukemia in pediatric patients is the precursor B-cell acute lymphoblastic leukemia (pre-B ALL) subtype. While treatment for pre-B ALL has improved greatly in recent years, there are still subtypes of leukemia that have poor prognosis and limited treatment options, and relapse remains a significant problem. Some of these subtypes can be characterized by specific genetic lesions. Mutations or deletions in the IKZF1 gene encoding the transcription factor Ikaros define one such high-risk subgroup. Ikaros is critical for lymphoid development and is recognized as an important tumor suppressor in pre-B ALL, in particular in BCR-ABL1+ (Ph+) pre-B ALL, where Ikaros loss-of-function mutations are found in greater than 80% of cases, and in the newly defined high-risk BCR-ABL1-negative ?Ph-like? subclass. In spite of the well- documented role of Ikaros as a tumor suppressor in pre-B ALL, there is limited understanding of how Ikaros functions to suppress leukemia. Furthermore, the molecular mechanisms underlying the strong association between the BCR-ABL1 oncogene and loss of Ikaros tumor suppressor function is poorly understood. Importantly, there are currently no therapies available that target Ikaros loss-of-function in pre-B ALL. We propose to study the unique features of Ikaros-mutated Ph+ leukemia in order to aid in rational design of targeted therapies, and we will focus on key aspects of Ikaros function. First, as Ikaros mutations are shown to result in aberrant expression of progenitor-restricted genes (including `stemness' genes), we will test the functional role of selected cell-surface proteins aberrantly expressed in Ikaros-mutated pre-B ALL, and evaluate their growth- or survival-promoting properties (Aim 1). Second, we will study the mechanism of Ikaros as a regulator of chromatin structure and ultimately investigate if epigenetic inhibitors can be employed to mimic Ikaros tumor suppressor function in IKZF1-mutated pre-B ALL (Aim 2). Furthermore, we will investigate the specific collaboration between the BCR-ABL1 oncogene and IKZF1-mutations by evaluating if Ph+ pre-B ALL has a specific chromatin accessibility signature enabeled by IKZF1-mutations that is required for the downstream BCR-ABL1 pathways (Aim 3). The results of this study may provide new functional targets for diagnosis and treatment of IKZF1-mutated pre-B ALL, and will provide critical insight into epigenetic dysregulation in leukemia for the rational use of epigenetic modulators for treatment.